A protester carries a photo of Maltese anti-corruption journalist Daphne Caruana Galizia, who was killed by a car bomb.
Photograph: Darrin Zammit Lupi/Reuters

A British law firm has been accused by the family of the murdered Maltese journalist Daphne Caruana Galizia of harassment, intimidation and an attempt to “cripple” her financially by threatening to sue her in the UK.

When Caruana Galizia was killed by a car bomb in October 2017, she was fighting 47 civil and criminal defamation lawsuits from an array of business people and politicians, brought by multiple law firms.

In the months before her death, the anti-corruption journalist received letters from the London office of the blue-chip firm Mishcon de Reya, which specialises in bringing defamation cases. Mishcon had been hired to defend the reputation of a client doing business in Malta.

“The firm sought to cripple her financially with libel action in UK courts,” Caruana Galizia’s three sons claim in a letter to the writers’ campaign group English PEN and seen by the Guardian. “Had our mother not been murdered, they would have succeeded.”

The case highlights what appears to be a trend: the use of London law firms by wealthy individuals and corporations to threaten foreign reporters with libel action in the UK, rather than in their home country.

It has triggered a debate at English PEN, which in December appointed Mishcon’s deputy chairman, Anthony Julius, to its board of trustees. The organisation’s mission is to defend writers and freedom of speech.

PEN has been campaigning for justice for Caruana Galizia. It marked the six months since her death with a letter to the European commission calling for better protections for whistleblowers and journalists, and condemning Malta’s prime minister for bringing a libel case against Caruana Galizia’s son Matthew.

The family have urged PEN to reconsider its decision to make Julius a trustee. One of Britain’s best known lawyers, he advised Diana, Princess of Wales, during her divorce and specialises in resolving disputes for ultra-wealthy families.

In their letter, the sons claim: “The campaign for justice in our mother’s case cannot be disentangled from the abuse that she suffered at the hands of Mishcon de Reya’s lawyers.

“The reputational damage to English PEN is only going to get worse for as long as [Julius] is there and his firm continues to harass and intimidate journalists.”

Both PEN and Julius have made it clear that he will remain on the board.

Julius said he believed a Mishcon lawyer could be a trustee of PEN. He has not been asked to step down. PEN said it would stop campaigning on Caruana Galizia’s case for the time being, because her family had withdrawn their cooperation in protest at Julius’s presence on the board.

Anthony Julius has made clear he plans to remain on the board of English PEN. Photograph: Graeme Robertson for the Guardian

The PEN director, Antonia Byatt, has ordered a review of how the organisation works with lawyers. A working group of trustees, including the Guardian associate editor, culture, Claire Armitstead, will devise a policy on the question. The working group will not include the two lawyers on the PEN board or its president, Philippe Sands QC.

PEN also plans to interview journalists and bloggers about their experiences with London libel firms.

In a statement published on Thursday, PEN said it was aware that libel threats remained a problem for investigative journalists. “We are therefore consulting with journalists – particularly those operating independently – on how the current law operates in practice.”

Mishcon was acting on behalf of the golden passports agent Henley and Partners, which has a contract with the Maltese government to market citizenship to wealthy investors.

Mishcon wrote to Caruana Galizia more than once in 2017 asking her to remove what it said were defamatory articles and reader comments, and factual inaccuracies about Henley.

Its letters make reference to British courts, stating, for example, that failure to take down material “will leave our client with little choice but to take legal action against you in the court of England and Wales”.

Mishcon said that under EU law, a complainant could bring defamation proceedings either in the state where the publisher was based, which allowed them to claim for damages to their reputation worldwide, or in any EU state where damage was suffered, in which case the payout only related to harm caused in that country.

The firm said that after she received their letter, Caruana Galizia had made corrections to her blog.

Henley said it was a British company, with its largest office in London. It had not sued Caruana Galizia, but politely asked her to remove material, and had a right to object to “fake news” and “defamatory insinuations”.

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Caruana Galizia’s sons said they were prompted to complain to PEN on 6 May after Mishcon made another threat of UK court action, on behalf of a different client, against the newspaper Malta Today.

“British law firms have a reputation for muscling in on journalists and silencing critics with the threat of ruinous court action,” Matthew Vella claimed.

He described the case as a Slapp threat, an allegation which Mishcon rejects. Slapp stands for Strategic Lawsuit Against Public Participation, a term for often baseless claims which aim to silence free speech and debate about issues of public interest by forcing critics to spend money defending themselves.

Mishcon claimed Malta Today had written a defamatory article without giving its client a chance to respond before publication. The firm said its response was proportionate, and was neither a jurisdictional threat nor a Slapp.

Vella said jurisdictional threats were being used to “condition” Maltese journalism, and that his paper has separately been threatened with UK court action by a Lebanese energy giant, a British biotech entrepreneur and a UK company with interests in football and shipping.

“If the Maltese courts uphold decisions delivered in a UK court on defamation actions, the consequences for the free press and Maltese democracy would be disastrous,” Vella claimed.

Such cases could be seen as a “jurisdictional tactic to silence the press without trial”, according to the academic Justin Borg-Barthet, who said the cost of defending suits was often enough to force small media houses to redact or remove articles.

Borg-Barthet, who is from Malta and lectures in the school of law at the University of Aberdeen, said a revision of EU law on jurisdiction in libel cases was urgently required to protect press freedoms.